Not much to say about this encounter, except Halion hits really hard (50 to 70k melee, plus specials)

P1, just nuke the boss, kite him away from the adds, enter portal.
P2, kite him around the room (you can blink through the wall for extra space, but Halion must never cross it or he will despawn). Avoid Twilight Cutters as much as possible.
P3, wait until his corporeality pushes him INTO the Twilight realm, so he takes more and more damage. Kite him just like P2, burn everything on cooldown.

Spec used : PoM, Temporal Shield, Cold Snap, Living Bomb, Incanter's Absorption
Glyphs : Combustion, Armors, Evocation.

Enjoy ! :)

Music : Fractal - Atrium
Armory : http://eu.battle.net/wow/en/character/hyjal/Araitik/advanced